I used to have that problem. I first purchased these little adhesive dots that reminded me of sand paper...but I hated those. Then, I found, at a local quilt shop, a plastic that you put on the underside of your rulers and they do not slip as easily. It's made especially for putting on your rulers so they won't slip. I'm sorry but I do not remember the name of the product. It came in a roll and you just cut the pieces you need for your rulers. Cut them about 1/4" shy from each edge for the most perfect fit! It's clear so it does not affect the ability to see through the rulers! Hope this helps!
Vanessa in Oklahoma